As background for understanding the present invention, it is noted that airguns have become recognized and accepted by seismic survey contractors and resource companies as being very useful.
In view of the importance to natural resource companies and to the nations of the world in obtaining seismic data from as deep within the earth as possible, and in view of the great costs involved in carrying out seismic surveying campaigns, there is continuing interest in the possibility of more powerful and even more reliable airguns. Moreover, very powerful and reliable airguns are desirable for other uses where intensely strong impulses may be employed.
In the description hereafter of the preferred embodiment of the airgun now conceived as being the best mode for placing this invention into practice, it is assumed that the reader has a familiarity with prior art airguns. If background information on airguns is desired, reference may be made to U.S. Pat. Nos. 3,249,177; 3,379,273; 3,310,128; 3,653,460; 3,808,822; 3,997,021; and 4,038,630.
As used herein, the term "pressurized gas" is intended to include any pressurized gas or pressurized mixture of gases which can be used in an airgun, for example such as compressed air, compressed carbon dioxide, compressed nitrogen, pressurized steam, and the like. In most cases, it is easier and cheaper to use compressed air, supplied from a mobile compressor or from portable tanks, and air is usually the preferred pressurized gas to be employed. However, in certain cases, for example, if the environment is suspected to contain combustible vapors or flammable gas or agents unduly reactive with air, then it may be desirable to use a non-combustion-supporting gas, such as carbon dioxide or nitrogen. Also "pressurized" is intended to mean at an elevated pressure substantially above atmospheric pressure, for example, such as in the preferred range from 500 pounds per square inch (p.s.i.) up to 6,000 p.s.i., although higher or lower pressure of the pressurized gas may occasionally be used.
The pressurized gas discharging apparatus described in the aforementioned patents have been referred to as airguns. An airgun is said to be "fired" when the pressurized air or other gas it contains is caused to be abruptly released and discharged with explosive-like suddenness into the surrounding environment.
The airguns known prior to the present invention generally include, as operative components, firing chamber means for holding gas under high pressure, a release means which can be actuated to abruptly release the pressurized gas through discharge ports, means for supplying gas under high pressure to the firing chamber means, and control means for causing the release means to be actuated to release the pressurized gas. The release means generally comprises a shuttle including a first (or releasing) piston positioned near the firing chamber for retaining the pressurized gas therein and a second (or operating) piston positioned in an operating cylinder or control chamber in which pressurized gas acts against the second piston to maintain it in "closed" position until it is to be released. The shuttle pistons are interconnected by a shaft which is reciprocally positioned between the operating cylinder and the firing chamber and is hollow for permitting a supply of pressurized gas to flow first to the operating cylinder and thence to the firing chamber.
The airguns may be self-firing by appropriate relative sizing of the exposed surface areas of the shuttle pistons or may be fired by operation of a solenoid valve for controlling flow of pressurized gas through various firing passages to cause the shuttle to be abruptly operated. A suitable solenoid valve means is described in U.S. Pat. No. 3,588,039. Both the self-firing and the solenoid-valve-controlled firing modes of airgun operation are described in my U.S. Pat. No. 3,379,273.
In such prior art airguns, the releasing piston of the shuttle in its closed position engages against an annular sealing assembly including a ring-shaped sealing member which is axially movable and is spring-biased to move upwardly toward the lower face of the releasing piston. This annular sealing assembly, including its spring means, spring retainer, the movable annular sealing member for engaging the lower face of the releasing piston and its associated slidable seal, is relatively complex and bulky. Moreover, this bulky annular sealing assembly surrounds the upper end of the firing chamber, thereby causing a restricted throat region above the firing chamber. Consequently, the pressurized gas being discharged from the firing chamber must pass through this restricted throat region, which tends to inhibit the desired rapidity of the gas outflow from the firing chamber, thereby limiting the powerfulness of the impulse.
The assembly of the main body portions of such prior art airguns has generally been accomplished by utilizing clamp rings clasping together flanged ends on the respective body portions. Such clamp rings produce an irregular exterior configuration for the airgun as a whole, and they tend to limit the overall strength and pressure capability of the airgun. Moreover, the clamp rings, being in an exposed exterior position on the airgun are subject to damage in the severe environmental conditions of being handled on the deck of a rolling and pitching vessel in a rough sea.